1. Field of the Invention
The present invention relates to a semiconductor optical device. More particularly, the present invention relates to a semiconductor optical device in which a plurality of optical devices integrated on a semiconductor substrate are electrically connected to each other by wire bonding.
2. Description of the Related Art
An electro-absorption modulator laser (EML) is a type of semiconductor optical device in which a laser generates oscillating light of a predetermined wavelength onto a semiconductor substrate, and in which an electro-absorption modulator for modulating the light oscillated by the laser are monolithically integrated through a semiconductor manufacturing process.
Several documents, including U.S. Pat. No. 6,057,954, which is filed on Sep. 18, 1998 by Parayanthal et al. and is entitled “Asymmetric Inductive Peaking for Optoelectronic Devices”, is hereby incorporated by reference as background material, and discloses in detail a semiconductor optical package structure in which the electro-absorption modulation laser and driving circuits for the electro-absorption modulation laser are electrically connected by wire bonding.
FIG. 1 illustrates a configuration of a conventional semiconductor optical package 100 including an electro-absorption modulation laser. The conventional semiconductor optical package comprises a semiconductor optical device 110, a submount 101 on an upper surface of which the semiconductor optical device 110 et al. are securely mounted, a driving circuit 106 that drives a semiconductor optical source (not shown), the driving circuit being positioned on one side of the submount 101, a signal line 103, an electrode 105 connected to a resistor 104, and respective first, second and third wires 120, 130, 140.
The semiconductor optical device 110 includes a semiconductor substrate 113, a semiconductor optical source (not shown) monolithically integrated on the semiconductor substrate 113, an optical modulator (not shown) and the like. The optical source for the semiconductor may include one of a distributed feedback semiconductor laser, etc.
The optical modulator may use an electro-absorption modulator for modulating an output of light generated by the semiconductor optical source or the like, and includes a first upper electrode 111 electrically connected to the signal line 103 and the resistor part 104. A common electrode 102 is formed between the semiconductor optical device 110 and the submount 101 to provide a common ground to the semiconductor optical source and the optical modulator, respectively.
A second upper electrode 112 is attached to an upper surface of the semiconductor optical source, and is electrically connected to the driving circuit 106 by the third wire 140.
The optical modulator may use an electro-absorption modulator for modulating light generated by the semiconductor optical source into an electrical signal or the like. A bandwidth characteristic of light that is modulated by the electro-absorption modulator significantly varies with capacitance, resistance and inductance of the electro-absorption modulator. Only when capacitance (<0.4 pf) and resistance of the electro-absorption modulator is negligibly small, does the bandwidth characteristic of light output from the semiconductor optical package not change considerably.
However, it is not possible to obtain the same values of capacitance (such as 4 pf) and a same amount of negligible resistance for every product in the manufacturing process of a semiconductor optical package including an electro-absorption modulation laser. As a result of this inability to produce products with nearly uniform a mounts of capacitance and resistance, optimal lengths of wires for connecting the electro-absorption modulation laser to the driving circuits thus vary according to the individual differences in capacitance, resistance of each product.
In an effort to circumvent the aforementioned problem, the conventional semiconductor optical package minimally maintains a length of the first wire (up to 0.3 mm; up to 0.5 nH) connecting the first upper electrode of the electro-absorption modulator to the signal line on the submount, and sets a relatively longer length of the second wire (1 to 2 mm) connecting the first upper electrode to a matching resistor on the submount. That is, the conventional semiconductor optical package can enhance a transmission characteristic of light and minimize loss by inducing an artificial peaking phenomenon in the electro-absorption modulator caused by using wires having specific lengths and/or inductances.
Consequently, the conventional semiconductor optical package improves the transmission characteristic of laser modulated light by increasing the length of the wire connecting the first upper electrode 111 to the matching resistor on the submount.
FIG. 2 shows an equivalent circuit illustrating inductance of the second wire 130 connected by wire bonding to the electro-absorption modulator of the semiconductor optical device 110 constituting the semiconductor optical package in FIG. 1, and FIGS. 3 to 6 are graphs showing bandwidth changes of light outputs according to inductance changes of the equivalent circuit in FIG. 2. Referring to FIG. 2, “L1” and “L2” shown in the equivalent circuit of the semiconductor optical package refer to the first wire 120 and the second wire 130, respectively, and “Cmod” and “Rs” refer to values of capacitance and series resistance of the electro-absorption modulator, respectively. Also, “RL” refers to resistance of the resistor part, and “Vs” and “R1” designate a high frequency voltage supply source and resistance of an electrical signal generator (not shown), respectively.
Transmission and bandwidth characteristics of light modulated by the electro-absorption modulator vary according to the individual capacitances and resistances of a specific electro-absorption modulator, and according to lengths and conductive properties of the first and second wires 120, 130 connected to the electro-absorption modulator by wire bonding. If the electro-absorption modulator has small values of capacitance (<0.4 pf) and resistance, the bandwidth characteristic does not vary with the change in length of the second wire 130. In actual production of electro-absorption modulator products, however, each individual modulator has slight variations in values of capacitance and/or resistance that normally occur according to slight changes in production conditions. To give a more optimized example, a length of the first wire 120 connecting the first upper electrode 111 of the electro-absorption modulator to the signal line 103 is set as 0.3 mm or less (inductance: 0.5 nH or less), and length of the second wire is set as 1 to 2 mm relatively longer than that of the first wire 120. In other words, the transmission characteristic of a modulated optical signal is improved and potential return loss is minimized by generating an artificial peaking in the electro-absorption modulator. The artificial peaking can be defined as a phenomenon in which a frequency or eye pattern characteristic of transmitted light exceeds a specific upper limit, and such a peaking phenomenon occurs when a length of the second wire 130 is excessively lengthened during the wire bonding of the second wire 130.
FIG. 3 illustrates a bandwidth characteristic of light modulated by the electro-absorption modulator when values of capacitance and resistance of the electro-absorption modulator are 0.7 pf and 10 Ω, respectively. FIG. 4 illustrates a bandwidth characteristic of light modulated by the electro-absorption modulator when values of capacitance and resistance of the electro-absorption modulator are 0.4 pf and 10 Ω, respectively. FIG. 5 illustrates a bandwidth characteristic of light modulated by the electro-absorption modulator when values of capacitance and resistance of the electro-absorption modulator are 0.7 pf and 15 Ω, respectively. FIG. 6 illustrates a bandwidth characteristic of light modulated by the electro-absorption modulator when values of capacitance and resistance of the electro-absorption modulator are 0.4 pf and 15 Ω, respectively.
As length of the second wire 130 is increased, the size of the submount is larger, resulting in a problem of necessitating an increase in the size of the semiconductor optical package. Also, due to the fact that the respective electro-absorption modulators have slight variations in value of capacitance or resistance according to the production conditions in practical production of electro-absorption modulator products, it is difficult to adjust a length of the second wire according to properties of each actual individual product.